Polycrystalline silicon has been a material used in semiconductor structures for some time. Conventionally, chemical vapor deposition techniques are employed where silane is decomposed in an elevated temperature onto the surface of a substrate, forming a layer of polycrystalline silicon, having a grain size in the range of from a few thousand angstroms to 2-3 microns, randomly oriented with respect to the original surface of the substrate. Typical prior art etching solutions for polycrystalline silicon include a solution of nitric acid and buffered hydrofluoric acid. Many problems arise employing prior art etching solutions such as this since its reaction rate is much too fast to provide a reasonable degree of control over the etching process. Prior art etching solutions evolve substantial quantities of heat in their exothermic reaction. Composite structures which include polycrystalline silicon, silicon dioxide and phosphosilicate glass cannot be differentially etched due to the substantially similar etching rates for the respective components by the prior art etchants. Other types of prior art etchants which have been employed for polycrystalline silicon include alkali metals which will contaminate the gate region of field effect transistor structures, for example. Still other prior art polycrystalline silicon etchants employ components which are not stable in air, for example pyrocatechol, is employed as a chelating agent in some prior art solutions and has a short shelf life because it oxidizes rapidly in open air. Still other prior art polycrystalline silicon etchant solutions require the maintenance of critical concentrations for their components, so that the solution must be repeatedly replenished during its use. A particular example of prior art anisotropic etching solutions for monocrystalline silicon is U.S. Pat. No. 3,738,881 to Erdman, et al.
Erdman, et al. teaches the anisotropic etching of monocrystalline silicon with an etchant comprised of an alkaline aqueous solution, an oxidizing agent, and passivating alcohol. Erdman mentions that quaternary ammonium hydroxides, as well as sodium and potassium hydroxides, may be employed in the process. Erdman's patent disclosure is limited to the etching of monocrystalline silicon where he wants to enhance the etching in the [100] direction and suppress etching of the high index phases 211, 311, 331, etc. so as to avoid rounded corners. He suppresses etching of the high index phases by adding alcohol to his solution. In contrast, the subject invention's major application is the etching of polycrystalline silicon isotropically. The 100 and the higher order 211, 311 and 331 phases of each crystallite will be etched by quaternary ammonium hydroxide etchants. If Erdman's solution were applied to etching polycrystalline silicon, those crystallites which do not have their [100] faces exposed, would not be etched at all since Erdman has required the presence of alcohol to suppress the etching of the 211, 311 and 331, etc. phases. Furthermore, Erdman discloses that he cannot etch monocrystalline silicon without the presence of an oxidant because of the formation of [111] pyramids on the 100 phases, which will terminate the etching process. The subject inventive etching solution does not use the oxident nor does it use the passivating alcohol, thereby enabling the solution to successfully etch the polycrystalline silicon isotropically. Erdmen never recognizes the problems of etching polycrystalline silicon.